Another cape insulation question

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The attached is a product evaluation, not a change in the code language. I would not approve or recommend leaving foam exposed based on that fact. Unless you use that exact foam noted and get your local official to confirm and sign off on a variance for it, I would plan on covering any installed foam products with a code approved ignition barrier.

Another cape insulation question

I gave you the site for SPF and the numbers sited for the other link, sorry. Here is what I understand; drywall on room side of knee wall, rigid foam on attic side, with vented attic separate from air flow to house or other attics- is acceptable under the 2006 IRC (haven’t checked for their up-dated version yet). 4.2.1.1; Attics; http://building.dow.com/ee/pdf/ESR-2142.pdf

Gary

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Another cape insulation question

Quote:

Originally Posted by GBR in WA

I gave you the site for SPF and the numbers sited for the other link, sorry. Here is what I understand; drywall on room side of knee wall, rigid foam on attic side, with vented attic separate from air flow to house or other attics- is acceptable under the 2006 IRC (haven’t checked for their up-dated version yet). 4.2.1.1; Attics; http://building.dow.com/ee/pdf/ESR-2142.pdf

Gary

I haven't had the time to read it all, but if the product in the evaluation report meets specific requirements noted in section R316.6, as listed below, then ok. If it doesn't then you either need a code variance or it needs a cover.

2009 IRC excerpts:

R316.5.3 Attics. The thermal barrier specified in Section R316.4 is not required where all of the following apply:

The above ignition barrier is not required where the foam plastic insulation has been tested in accordance with Section R316.6.

R316.6 Specific Approval. Foam plastic not meeting the requirements of Sections R316.3 through R316.5 shall be specifically approved on the basis of one of the following approved tests: NFPA 286 with the acceptance criteria of Section R302.9.4, FM4880, UL 723, UL 1040 or UL 1715, or fire tests related to actual end-use configurations. The specific approval shall be based on the actual end use configuration and shall be performed on the finished foam plastic assembly in the maximum thickness intended for use. Assemblies tested shall include seams, joints and other typical details used in the installation of the assembly and shall be tested in the manner intended for use.

- foam sandwiched in a typical wall is OK
- used in an attic/knee wall, the foil backed foam w/ foil side exposed
- add 1/4" plywood skin over the foam

Does that seem about right?

Is the foam considered a vapor barrier? Meaning, if it were used on the knee-wall side of these attics, would it cause a problem? I'm having R15 FG batts installed on my knee walls, I'm considering applying the foil backed foam board over it to act as an air barrier and increase the insulation by ~R5. Or, is using foam in at situation bad because it would create a moisture/vapor barrier at the wrong side of the wall?

Another cape insulation question

- foam sandwiched in a typical wall is OK
- used in an attic/knee wall, the foil backed foam w/ foil side exposed
- add 1/4" plywood skin over the foam

Does that seem about right?

Is the foam considered a vapor barrier? Meaning, if it were used on the knee-wall side of these attics, would it cause a problem? I'm having R15 FG batts installed on my knee walls, I'm considering applying the foil backed foam board over it to act as an air barrier and increase the insulation by ~R5. Or, is using foam in at situation bad because it would create a moisture/vapor barrier at the wrong side of the wall?

There is a flip side to the foil facing. The current code for CT allows for the vapor retarder to be on the exterior of the envelope if the continuous foam layer is R-10 or greater. In that case you wouldn't need the vapor retarder at the interior face anymore.

If you already have the vapor barrier against the sheetrock, then you would want to drop the foil facing. But you can still do a foam layer depending on the type and thickness of foam. XPS is an acceptable foam at 1" with an R-5 and perm of 1.5. EPS is very permeable and can get much thicker. I'm not sure how much polyiso, but you'd be better off with XPS if the vapor retarding is a factor.

So, yes your assembly as noted looks good, though the foil facing is dependent on your existing vapor retarder status and the plywood is most likely needed unless you can find the foam that meets all those criteria.

Another cape insulation question

You could use unfaced foam board on the attic side without an ignition barrier (plywood) as you already have the thermal barrier (drywall) to give it the 15 minute required by Code if you meet the conditions: http://dow-styrofoam.custhelp.com/ap...KkNueDl5R2s%3D
Always check with AHJ.

Gary

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